1. Field of the Invention
The invention relates in general to a method of fabricating a semiconductor device, and more particularly, to a method of fabricating a thin film resistor of an interconnect by dual damascene.
2. Description of the Related Art
Dual damascene is a technique which fabricates planar and vertical interconnects at the same time. An insulation layer is formed on a substrate. After planarization, the pattern of conductive wires and contact window is transferred. The insulation layer is then etched to form a trench for the planar interconnects, and a contact window for vertical interconnects. A metal layer is formed to fill the trench and the contact window to form the conductive wires and the contact at the same time. The interconnection is thus achieved. Silicon has a certain solid solubility for metal at a high temperature. While a high temperature process is performed, a mutual diffusion occurs between silicon atoms and metal atoms to cause spikings. The spikings are often too long to cause a short circuit by piercing through the silicon. The short circuit affects the performance of devices greatly, and sometimes even causes a device failure. Therefore, a barrier layer is formed between a metal layer and a silicon layer to avoid the spiking effect, as well as to enhance the adhesion between silicon and metal.
A resistor is a component commonly used in both memory and logic circuit. The resistance of a resistor is a function of both the length and cross sectional surface area, that is, R=.rho..sup.L/A, wherein, .rho. is the resistivity, L is the length through which current flowing, and A is the cross sectional surface area which current flowing through.
A resistor in an integrated circuit is typically formed by lightly doping a polysilicon region. Strips of polysilicon with various length and cross sections are formed as resistors with different resistance. Alternatively, resistors with various different resistance may also be formed by high resistant conductor and low resistant conductor. Typically, the low resistant conductor is formed by doped polysilicon, while the high resistant conductor is formed undoped polysilicon. As the integration of a semiconductor device is increased, the quality demand of material for forming the semiconductor device is raised. For example, to obtain a device with a reduced surface or volume, a resistor with a sufficiently high resistance has to be formed with a restricted dimension. However, the resistance per unit area and length of material such as polysilicon is limited, and thus, causes the difficulty for the fabrication of device with a high integration.
To increase the resistance per unit of a resistor, material such as chromium silicide (Cr.sub.x Si.sub.y) has been used to fabricate resistant layer to replace the doped or undoped polysilicon.
FIG. 1A to FIG. 1C shows a fabrication method for forming a thin film resistor.
In FIG. 1A, a substrate 100 is provided. A borophosphosilicate glass (BPSG) layer 102 is formed on the substrate 100. A thin film resistant layer 104 is formed on the BPSG layer 102. Using sputtering, a metal layer made of aluminum or alloy of aluminum/silicon/copper (Al/Si/Cu) 106 is formed on the BPSG layer 104 to protect the thin film resistant layer 104 from being etched in a subsequent dry etching process.
In FIG. 1B, the metal layer 106 is patterned, and the remaining metal layer 106a is formed as an etching mask for defining the underlying thin film resistant layer 104. The defined thin film resistant layer is denoted as 104a. The BPSG layer 102 is defined by dry etching to form a contact window 110 penetrating through the resultant BPSG layer 102a. A conductive layer 112 is formed over the substrate 100 and to fill the contact window 110.
In FIG. 1C, the conductive layer 112 is patterned as a conductive layer 112a of a conductive wire for interconnection 112a. The metal layer 106a is stripped to expose the thin film resistant layer 104a. After the formation of the thin film resistant layer 104a, an electric characteristic test is typically performed. A laser cutting machine is often used to cut the thin film resistant 104a into a thin film resistor with a required resistance. Therefore, the requirement of the circuit design can be more precisely achieved.
However, in the conventional method, an aluminum contained metal layer 106 is formed as a protection layer as the thin film resistant layer 104a. While a barrier layer is formed by material containing titanium, the aluminum in the protection layer will react with titanium. As a consequence, the protection layer cannot effectively protect the thin film resistant layer 104a. A conformal barrier for improving the adhesion between a contact window and a contact plug and to avoid the spike effect can not be formed in the contact window.